The present invention relates generally to circuits for processing received digital signals and particularly concerns circuits for performing the conditional access and transport demultiplexing functions of a digital signal television receiver.
Conditional access systems for subscriber units such as cable television subscriber set-top box decoders are well-known in the art. Conditional access is conventionally achieved by downloading one or more authorization levels for storage in the decoder. The stored authorization levels may comprise a stored bit map or a list of stored individual multi-bit codes, or a combination of both. Each received subscription program, which is normally scrambled or encrypted to prevent access thereto by an unauthorized subscriber, includes an authorization code (sometimes referred to as a program tag) identifying the associated program. If the program tag corresponds to an authorized level in the stored bit map memory or to a stored listed authorization level of the subscriber, a descrambling or decryption circuit within the subscriber's decoder is enabled to descramble or decrypt the signal for viewing by the subscriber. On the other hand, if the received program tag does not match any stored authorization level descrambling or decryption of the accompanying program is inhibited.
Depending on the desired resolution, recent advances in technology have made possible the transmission and reception of one or more digitally compressed television signals over a single 6 MHz television channel. The television signal is preferably compressed and arranged for transport in accordance with international standards established by the Moving Pictures Expert Group (MPEG). In accordance with the MPEG standard, the compressed digital television information is arranged for transmission in the form of a multiplexed transport stream of fixed length MPEG packets including, for example, video packets, audio packets and conditional access packets (all packets other than conditional access packets being referred to as product packets). Each packet in the transport stream includes a 4-byte header comprising a 13-bit packet identification code (PID) identifying the so-called payload (184 bytes) of the respective packet. A PID having a value equal to one (i.e. 00 . . . 1) has been reserved for conditional access packets.
In a general sense, conditional access for digitally compressed subscription systems of the foregoing type may be achieved using techniques quite similar to those employed in prior art analog subscription systems. In addition, the received transport stream must be demultiplexed to separate the conditional access packets and the various product packets; e.g. video and audio packets. The separated conditional access packets are ultimately used to control the conditional access system of the set-top decoder while the separated video and audio packets are supplied to respective decompression circuits which provide reproducible video and audio signals. The functions of conditional access and transport stream demultiplexing have traditionally been conceived as independent operations and have been effected by respective dedicated integrated circuits. However, since a number of similar operations must be performed in order to execute both functions, it is believed that a single circuit optimized for performing both functions through selected shared operations would be a much more efficient technique.